Static IV/nonce (ARCH2114)

Key python.security.static-iv · Severity error · Category security · CWE CWE-329

A constant or zero IV/nonce destroys the semantic security of CBC/CTR/GCM modes and causes critical breaks when reused with the same key. Generate a random IV per message.

Incorrect

from cryptography.hazmat.primitives.ciphers.aead import AESGCM

def encrypt(key, data):
    nonce = b"\x00" * 12
    return AESGCM(key).encrypt(nonce, data, None)

Correct

import os
from cryptography.hazmat.primitives.ciphers.aead import AESGCM

def encrypt(key, data):
    nonce = os.urandom(12)
    return nonce + AESGCM(key).encrypt(nonce, data, None)

Fix

Generate a cryptographically random, unique IV/nonce for every encryption operation.